A panel of monoclonal antibodies has identified four serogroups of xenotropic murine retroviruses. We have used these antibodies to serologically and structurally characterize the viruses expressed by lymphohematopoietic tissues. All xenotropic viruses recovered from spleen, bone marrow and embryonic liver belong to group 1. The predominant cell membrane gp70 expressed by spleen and bone marrow cells also belongs to group 1 whereas that expressed by a large population (greater than 60%) of early embryonic liver cells resembles serologically and structurally group 4 viruses. It is this gp70 which is expressed by erythroid cells during an intermediate stage of normal differentiation. This antigen has also been identified on some but not all erythroid cell lines. However, several attempts to recover infectious group 4 virus from these cell populations (including the erythroid cell lines) have been unsuccessful. This suggests that unlike group 1, group 4 gp70 is not encoded by a replication competent provirus. We now have preliminary evidence in a serologic survey of many different hematopoietic cell lines that myeloid specific retroviral products may exist. The use of these antibodies specific for endogenous viruses is revealing a serologic heterogeneity not previously appreciated and may provide a tool for identifying genetic loci which are functional in normal cellular differentiation.